1. Field
Example embodiments generally relate to an optical scanning apparatus, an image forming apparatus including the same and a method of regulating the optical scanning apparatus.
2. Discussion of the Background
Image forming apparatuses such as a laser beam printer, a digital copier, a laser facsimile are equipped with an optical scanning apparatus which writes a latent image on an image carrier or a photoreceptor.
Such an optical scanning apparatus is structured such that, for example, a light flux or an light beam from an optical source is deflected by a rotary deflector so that the photoreceptor is exposed and scanned by the light flux or the light beam.
There is a demand for an image forming apparatus that is capable of handling various types of paper. An image forming apparatus, which may accommodate not only a sheet of commonly used paper, but also a post card, release coated paper, thin paper such as tracing paper and so forth, is desired by users.
In printing processing of an electrophotographic-type image forming apparatus, a latent image is formed on an image carrier by an optical scanning apparatus. The developing mechanism visualizes the latent image with toner, and produces a toner image.
Subsequently, the toner image is transferred onto a printable medium such as a transfer sheet and is fixed. Then, the transfer sheet is ejected outside the image forming apparatus.
Generally, fixing the toner image on a sheet of paper having a relatively large thickness requires greater heat amount when compared with fixing on standard paper. Consequently, a process linear velocity and a printing speed are reduced so that the heat amount per unit of time is increased, thereby securing fixability.
As a result, when printing on relatively thick paper, the printing speed is reduced and the productivity is also reduced.
In order to accommodate an increasing need for color printing in recent years, a color image forming apparatus which forms a full-color image by overlaying four different colors of toner, black (Bk), magenta (M), cyan (C) and yellow (Y) is known. Such an image forming apparatus also requires a larger heat amount for fixing an image when compared with a single-color (e.g., black) image forming apparatus, because four different colors are overlaid on one another.
In the case of color image forming apparatus, when a full-color image is formed, the process linear velocity is reduced. In other words, the productivity is reduced when compared with a single-color (black) image forming apparatus.
In order to accommodate various demands of users, an image forming apparatus normally has a plurality of process linear velocities so that the linear velocity may be changed according to different modes such as paper types and switching between a monochrome mode and a color mode.
When the process linear velocity is reduced so as to reduce the printing speed, the number of rotations or the revolution number the rotary deflector may also be reduced by a ratio relative to a default linear velocity. On the contrary, when the range of the process linear velocity V [mm/sec] is increased, the range of the revolution number of the rotary deflector will be increased, accordingly.
A DC brushless motor is commonly used for a motor used in the rotary deflector. Depending on an optimal range of input clock to the motor or the types of shaft bearing or shaft bearing structures, an optimal revolution number is already predetermined to some extent.
When the motor is operated out of the optimal range of revolution number, the characteristics of the motor may not be satisfied.
In a case where the motor is rotated significantly less than the optimal range of the revolution number, it is known that low frequency jitter or a rotation variation may be worsened. As a result, an image distortion such as a fluctuation image may be generated.
In a case where the motor is rotated beyond the optimal range of the revolution number, problems with the product life of the motor itself, heat generation and noise may be generated.
In light of this, it is desired that the rotary deflector includes an optimal revolution number and is used within a range of the revolution number as minimum range as possible. Thereby, an effect in which the time required for switching the revolution number is reduced may be attained, for example.
In addition, in a case where the process linear velocity is changed so as to keep an exposure energy on the photoreceptor per unit of time at a constant level, it may be necessary to change an amount of exposure light onto the photoreceptor to form a latent image on the image carrier, accordingly.
In other words, it may be necessary to change the output of the laser beam emission from the light source according to the change in the process linear velocity.
Normally, when a constant k which is obtained based on an optical specification of the scan lens disposed in the optical scanning apparatus, an exposure sensitivity and a necessary exposure width of the photoreceptor, are fixed, the exposure light amount P onto the photoreceptor is given by: P=k×V/N, where V[mm/sec] is a linear velocity, and N is the number of light sources.
Similar to the revolution number of the rotary deflector, when the range of the linear velocity V[mm/sec] is wide, the range of the amount of the exposure light may become also wide.
A laser diode used as an optical source, for example, may not be able to stably emit light in an output range less than or equal to the 15% of a rated output. Consequently, it is possible to generate problems associated with a beamspot diameter and an LD modulation. As a result, an image deviation may occur.
Furthermore, when the laser diode is used at the limit of the rated output, it may accelerate the deterioration of the product life of the laser diode. Thus, the reliability of the entire apparatus may also be deteriorated.
In such an optical scanning apparatus as described above, there is normally provided a synchronous detector which regulates a writing start position in a main scan direction relative to an image carrier. Generally, while the laser beam emitted from the light source is deflected and scanned by the rotary deflector, the laser beam is led to the synchronous detector positioned outside the image formation area so that timing is electronically met, and the writing start position of the main scan direction is regulated.
The synchronous detector equipped with a photo IC for receiving light on a printed circuit board is common. However, the photo IC has a characteristic in which the output varies depending on the amount of the incident light striking the light receiving surface.
In a case where the amount of the incident light varies, the timing of the writing start position may not be constant which may cause image deviation due to the image deviation, deterioration of the accuracy of color alignment and/or deterioration of reproduction of color during full-color image formation.
Furthermore, an output of a photo IC may vary depending on the scan speed of the light beam. In a case where the revolution number of the rotary deflector is changed according to the change in the process linear velocity, the output of the photo IC may also vary. Thus, as described above, similar to the case in which the amount of the incident light to the photo IC may change, the timing of the writing start position may not be constant, thereby inducing an abnormal image.
In order to accommodate recent growing needs for higher speed operation, a multi-beam optical scanning apparatus is often used in image forming apparatuses such as a printer, a facsimile and a copier. The laser diode to be used in the optical source, for example, has a lifetime longer than a mechanical lifetime, and thus a longer operation than the apparatus is assured.
However, there is a possibility that the laser diode may fail or deteriorate early so that the operation is impaired. If the entire image forming apparatus fails to operate, printing operation is discontinued until a repairman comes to fix the problem. Furthermore, a maintenance fee and a labor cost and the like may increase.